1. Field of the Invention
The present invention relates to an image forming device and an image forming method, and more particularly to an image forming device and an image forming method with which image formation is carried out in accordance with a specified resolution by irradiating a recording medium, which is wound on a peripheral surface of a cylindrical drum, with a plurality of light beams along a sub-scanning direction parallel to a rotation axis of the drum.
2. Description of the Related Art
Conventionally, image forming devices have been utilized in which an image is recorded on a recording medium, which is wound on a peripheral surface of a cylindrical drum, using a number n of light beams, which are disposed to be separated by predetermined intervals along a sub-scanning direction. In such an image forming device, the drum rotates oppositely to a main scanning direction, and an optical head for irradiating the light beams moves along the sub-scanning direction. The optical head forms the image by moving in the sub-scanning direction only by a distance corresponding to the length of an image that is formed by the light beams during one rotation of the drum. Accordingly, the optical head proceeds to form a two-dimensional image in a spiral (helical) form. Consequently, as the image is formed in the spiral form, a problem arises in that the formed image is distorted to a parallelogram shape, as shown in FIG. 11.
Therefore, in order to eliminate the distortion described above, there is a method for forming images in which the recording medium is preparatorily inclined and wrapped on the drum at an angle in accordance with the number of light beams and the resolution.
Further, a multi-beam rendering method has been disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-43318. This disclosure proposes a method for recording in which, during recording of one line in the main scanning direction, recording pixel is shifted by a maximum of n in a direction opposite to a moving direction of the optical head.
However, in the method in which the recording material is inclined beforehand and fixed to the drum in accordance with the number of light beams and the resolution, the inclination cannot be changed in accordance with a change in resolution. Therefore, there is a problem in that the resolution is fixed and image forming cannot proceed in accordance with another resolution.
Further, in the multi-beam rendering method disclosed in JP-A 2000-43318, because the image is formed by moving it in the sub-scanning direction precisely by a distance corresponding to a sub-scanning direction length of image formed by the light beam, as the number of light beams increases, the number of times the recording pixels are shifted increases. Thus, there is a problem in that linear flaws in the main scanning direction occur and image quality deteriorates.
The present invention is provided to solve the above-described problems and an object of the present invention is to provide an image forming device and an image forming method respectively capable of implementing a change of resolution and capable of forming an image with few linear flaws in a main scanning direction.
In order to solve the problems described above, an image forming device of a first aspect of the invention is an image forming device for forming an image in accordance with an inputted resolution by irradiating a recording material, the image forming device comprising: a rotary drum including a peripheral surface, the rotary drum rotating in a main scanning direction in a state in which the recording material is wound onto the peripheral surface; a recording head disposed at a portion facing the peripheral surface of the rotary drum, the recording head being moveable in an axial direction of the rotary drum, which is a sub-scanning direction, the recording head being capable of irradiating with a plurality of light beams which are separated by predetermined intervals in the sub-scanning direction, and the recording head irradiating the recording material with the light beams on the basis of image data while the rotary drum is rotating; and an image data-shifting section which shifts image data, wherein the recording material is wound onto the rotary drum in an incline manner such that, in a case of recording at a predetermined resolution, a main scanning line to be recorded is parallel to an end portion of the recording material which is along a direction of winding onto the rotary drum, and in a case of recording at the inputted resolution other than the predetermined resolution, the image data-shifting section, at each occurrence of displacement of a main scanning line being recorded at the recording material, relative to a main scanning line recorded at the predetermined resolution, by a predetermined pixel amount determined in advance, shifts image data by the predetermined pixel amount in a direction opposite to the direction of the displacement.
Further, an image forming method of a second aspect of the invention is a method for forming an image with an image forming device in accordance with an inputted resolution by irradiating a recording material, the image forming device comprising a rotary drum including a peripheral surface, the rotary drum rotating in a main scanning direction in a state in which the recording material is wound onto the peripheral surface, and a recording head disposed at a portion facing the peripheral surface of the rotary drum, the recording head being moveable in an axial direction of the rotary drum, which is a sub-scanning direction, the recording head being capable of irradiating with a plurality of light beams which are separated by predetermined intervals in the sub-scanning direction, and the recording head irradiating the recording material with the light beams on the basis of image data while the rotary drum is rotating, the method comprising the steps of: winding the recording material onto the rotary drum in an incline manner such that, in a case of recording at a predetermined resolution, a main scanning line to be recorded is parallel to an end portion of the recording material which is along a direction of winding onto the rotary drum, and in a case of recording at the inputted resolution other than the predetermined resolution, at each occurrence of displacement of a main scanning line being recorded at the recording material, relative to a main scanning line recorded at the predetermined resolution, by a predetermined pixel amount determined in advance, shifting image data by the predetermined pixel amount in a direction opposite to the direction of the displacement.
According to the first and second aspects of the invention, the rotary drum and the recording head are provided. The rotary drum rotates in the main scanning direction with the recording material wound at the peripheral surface. The recording head is disposed at a portion facing the peripheral surface of the rotary drum, is capable of emitting the plurality of light beams, and is moveable in the sub-scanning direction, which is the axial direction of the rotary drum. The light beams are separated by predetermined intervals along the sub-scanning direction. While the drum rotates, the light beams are irradiated at the recording medium from the recording head on the basis of the image data. Thus, an image is formed in accordance with the inputted resolution. When an image is formed in this way, the image is formed in a helical form. Therefore, main-scanning lines are formed with displacement in the sub-scanning direction, and are inclined with respect to the recording medium to a certain extent (see FIG. 11).
Therefore, in the first and second aspects of the invention, the recording material is slantedly wound such that, if an image is recorded at the predetermined resolution, the main scanning lines to be recorded are parallel with both ends (edges) of the recording material along the direction of winding onto the drum. In addition, for each occasion on which a main scanning line being recorded on the recording material is displaced, relative to the main scanning line that would be recorded at the predetermined resolution, by an amount corresponding to a predetermined number of pixels which is set in advance, image data is shifted the amount corresponding to the predetermined number of pixels in the direction opposite to the direction of displacement. Consequently, because the image is formed with the recording material being wound onto the rotary drum at an angle and the image data being shifted the amount corresponding to the predetermined number of pixels in the direction opposite to the direction of displacement whenever the main scanning lines being recorded on the recording material are displaced relative to the main scanning lines that would be recorded at the predetermined resolution by the amount corresponding to the predetermined number of pixels, the number of times data is shifted can be made smaller than in a case in which data is compensated by shifting but the recording material is wound on the drum non-inclinedly, and higher quality images can be formed. Further, because the number of times the data is shifted can be made smaller, loading on the device can be alleviated.
Moreover, with compensation by merely winding a recording material at an angle in accordance with a predetermined resolution, the resolution for which compensation is possible would be fixed. However, because in the present invention compensation is performed also by data shifting, compensation can be implemented in accordance with a change in resolution.
For judging displacement of the amount corresponding to the predetermined number of pixels, a one-line length in the main scanning direction is calculated from the predetermined resolution, the inputted resolution, the number of light beams, and a circumferential length of the rotary drum. It can be determined where there is displacement of the amount corresponding to the predetermined number of pixels by recording the calculated main scanning direction one-line length.
Furthermore, in the first and second aspects of the present invention, the predetermined resolution may be set to a minimum specifiable resolution, and the recording material wound onto the rotary drum at an angle in accordance with this minimum resolution. Because pixel intervals at low resolutions are wider than at high resolutions, steps caused by image data shifts when forming an image at low resolution will be larger and will cause image quality to deteriorate. Accordingly, the recording material is wound on the drum at the angle corresponding to the minimum resolution, at which the steps are largest, and compensation is performed by data shifts. Thus, the number of data shifts at low resolutions, at which the steps are more obvious, can be reduced. The number of shifts increases as the resolution gets higher. At higher resolutions, the steps are less obvious. Consequently, images of higher image quality can be formed.
Furthermore, in the first and second aspects of the present invention, a number of times of shifting of image data and a data length in the main scanning direction of the shifted image data in one main scanning are determined on the basis of the predetermined resolution, the inputted resolution, the number of the light beams, and a circumferential length of the rotary drum.